2. Field of the Invention
The present invention relates to a power supply unit suitable for use in an electrostatic printing apparatus.
2. Brief Description of the Prior Art
Prior art power supply units of this type include one which uses a ferro-resonance transformer and one which uses a self-excited or externally excited inverter. The power supply unit which uses the ferro-resonance transformer is large in size and heavy in weight and the power supply unit which uses the self-excited o externally excited inverter has a large switching loss and is extremely noisey. When such a power supply unit is used to drive a plurality of chargers of a copying machine, one transformer for each of the chargers is necessary if the chargers are to be driven with constant currents. When low voltage loads such as a developing bias circuit and a sequence control circuit are to be fed in addition to the high voltages fed to the chargers, a plurality of transformers are also necessary. As a result, the number of transformers needed increases and hence the manufacturing cost increases too much.
In the prior art, in order to form an image on a photosensitive drum, a sequence controller for controlling an entire system is operated after the power is turned on to control timing of the photosensitive drum and timing of energization of the chargers to generate various high voltages in order to control the chargers such that an optimum image formation condition is attained. Since an on-off control circuit for a motor and an on-off control circuit for the high voltages to be applied to the chargers are necessary in order to control the speed of the rotary drum and the charge quantities of the chargers, the unit is complex and hence expensive.